European patent document no. 1 138 915 refers to a method in which, during charging of a piezoactuator of an injector, the transferred quantity of electrical charge can be determined. The corresponding quantity of electrical charge transferred during discharging of the piezoactuator can likewise be determined. This is accomplished by integration of a current signal. In order to reduce errors upon integration of the current signal and thereby to increase the precision with which the transferred charge quantity is ascertained, an alignment of the integration process, to be performed at specific points in time, is proposed. This alignment is to be performed, in particular, when the internal combustion engine is started. The reason for this is that ordinary control unit concepts and output stage concepts can operate only sequentially, so that an alignment cannot occur during triggering of the output stage or the piezoactuator.